1. Field of the Invention
The present invention relates to an organic light emitting display, and more particularly, to an organic light emitting display which compensates a threshold voltage of a driving transistor to improve a uniformity in brightness.
2. Discussion of Related Art
Recently, various flat panel displays, which are thinner than a comparable cathode ray tube display, have been developed. As a flat panel display, an organic light emitting display has excellent emission efficiency, brightness and viewing angle, as well as a fast response time.
An organic light emitting diode (OLED) of an organic light emitting display has a structure that includes an emitting layer for emitting light interposed between a cathode electrode and an anode electrode. An electron and a hole are injected into the emitting layer and recombined in the emitting layer, so that an exciton is created and light is emitted when the exciton is transitioned to a low energy band.
FIG. 1 is a circuit diagram of a pixel in a conventional organic light emitting display. Referring to FIG. 1, the pixel includes an organic light emitting diode OLED, a driving transistor M2, a storage capacitor Cst, and a switching transistor M1. Further, a scan line Sn, a data line Dm, and a power source line Vdd are connected to the pixel. For reference, n is an arbitrary integer between 1 and N, and m is an arbitrary integer between 1 and M.
The switching transistor M1 includes a source electrode connected to the data line Dm, a drain electrode connected to a first node A, and a gate electrode connected to the scan line Sn.
The driving transistor M2 includes a source electrode connected to the power source line Vdd, a drain electrode connected to the organic light emitting diode OLED, and a gate electrode connected to the first node A. Here, a current required for emitting light is applied to the organic light emitting diode (OLED) in response to a signal inputted to the gate electrode of the driving transistor M2. Further, an intensity of the current applied to the driving transistor M2 is controlled by a data signal transmitted through the switching transistor M1.
A storage capacitor Cst includes a first electrode connected to the source electrode of the driving transistor M2, and a second electrode connected to the first node A, and is employed to maintain the voltage applied between the source electrode and the gate electrode of the driving transistor M2 for a predetermined period.
In operation, when the switching transistor M1 is turned on in response to a scan signal transmitted to the gate electrode of the switching transistor M1, the storage capacitor Cst is charged with a voltage corresponding to the data signal, and the voltage charged in the storage capacitor Cst is applied to the gate electrode of the driving transistor M2, so that the current flows through the driving transistor M2, thereby allowing the organic light emitting diode OLED to emit light.
At this time, the current flowing into the organic light emitting diode OLED from the driving transistor M2 is obtained by the following equation 1.
                              I          OLED                =                                            β              2                        ⁢                                          (                                  Vgs                  -                  Vth                                )                            2                                =                                    β              2                        ⁢                                          (                                  Vdd                  -                  Vdata                  -                                                          Vth                                                                      )                            2                                                          [                  equation          ⁢                                          ⁢          1                ]            
where, IOLED is a current flowing into the organic light emitting diode OLED; Vgs is a voltage applied between the source and gate electrodes of the driving transistor M2; Vth is the threshold voltage of the driving transistor M2; Vdd is a voltage source for the pixel; Vdata is a voltage of the data signal; and β is a gain factor of the driving transistor M2.
Referring to equation 1, the current IOLED is related to the threshold voltage Vth of the driving transistor M2.
However, when a conventional organic light emitting display is fabricated, a deviation may arise between threshold voltages of driving transistors (e.g., the driving transistor M2). The deviation between the threshold voltages of the driving transistors causes the intensity of currents flowing into organic light emitting diodes OLEDs to be not uniform, so that the conventional organic light emitting display displays an image with non-uniform brightness.